pubs.acs.org/Langmuir © 2009 American Chemical Society
Formation Mechanism and Mechanics of Dip-Pen Nanolithography Using Molecular Dynamics Cheng-Da Wu,† Te-Hua Fang,*,† and Jen-Fin Lin‡ †
Institute of Mechanical and Electromechanical Engineering, National Formosa University, Yunlin 632, Taiwan, and ‡Department of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan Received August 7, 2009. Revised Manuscript Received September 23, 2009
Molecular dynamics simulations are used to investigate the mechanisms of molecular transference, pattern formation, and mechanical behavior in the dip-pen nanolithography (DPN) process. The effects of deposition temperature were studied using molecular trajectories, the meniscus characteristic, surface absorbed energy, and pattern formation analysis. At the first transferred stage (at the initial indentation depth), the conformation of SAM molecules lies almost on the substrate surface. The molecules start to stand on the substrate due to the pull and drag forces at the second transferred stage (after the tip is pulled up). According to the absorbed energy behavior, the second transferred stage has larger transferred amounts and the transfer rate is strongly related to temperature. When molecules were deposited at low temperature (e.g., room temperature), the pattern shape was more highly concentrated. The pattern shape at high temperatures expanded and the area increased because of good molecular diffusion.
1. Introduction Lithographic methods are used in microfabrication, nanotechnology, and molecular electronics fields. These methods often rely on the patterning of a resistive film, followed by a chemical etching of the substrate. Dip-pen nanolithography (DPN)1 is a great scanning probe nanopatterning technique in which an atomic force microscope (AFM) tip is used to deliver molecules to a surface via a solvent meniscus, which naturally forms in the ambient atmosphere. This direct-write technique offers highresolution patterning for a number of molecular inks on a variety of substrates.2-6 One of the most important attributes of DPN is that patterns of multiple molecular inks can be formed or aligned on the same substrate since the same device is used to image and write a pattern. Despite the widespread application of DPN,7-10 little is known about the molecular mechanisms of transport, growth, and mechanics in DPN. Most of these fundamental aspects are extremely difficult to obtain from experiments. Therefore, several theoretical models have been proposed to explain the dynamics of DPN.11,12 These theories could be successfully used to predict some pattern characteristic. However they lack molecular foundation and hardly reveal the real-time dynamics of DPN, because of complex physic and chemical interaction between molecule-molecule and
molecule-substrate. In the present study, molecular dynamics (MD) simulation is used to reveal the mechanisms and dynamics of monolayer growth in DPN. MD simulation is an effective tool for studying material behavior at the nanometer scale; it provides detailed deformation information at the atomic level. Many nanosystems have been
*Corresponding author. E-mail:
[email protected]. Telephone: þ886-5-6315395. Fax: þ886-5-6315397.
(1) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 203, 661. (2) Mirkin, C. A. ACS Nano 2007, 1, 79. (3) Jaschke, M.; Butt, H. J. Langmuir 1995, 11, 1061. (4) Noy, A.; Miller, A. E.; Klare, J. E.; Weeks, B. L.; Woods, B. W.; DeYoreo J. J. Nano Lett. 2002, 2, 109. (5) Nafday, O. A.; Vaughn, M. W.; Weeks, B. L. J. Chem. Phys. 2006,125, 144703. (6) Cho, Y.; Ivanisevic, A. Langmuir 2006, 22, 8670. (7) Ginger, D. S.; Zhang, H.; Mirkin, C. A. Angew. Chem., Int. Ed. 2004, 43, 30. (8) Wilson, D. L.; Martin, R.; Hong, S.; Cronin-Golomb, M.; Mirkin, C. A.; Kaplan, D. L. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 13660. (9) Su, M.; Dravid, V. P. Appl. Phys. Lett. 2002, 80, 4434. (10) Maynor, B. W.; Li, J.; Lu, C.; Liu, J. J. Am. Chem. Soc. 2004, 126, 6409. (11) Sheehan, P. E.; Whitman, L. J. Phys. Rev. Lett. 2002, 88, 156104. (12) Jang, J.; Hong, S.; Schatz, G. C.; Ratner, M. A. J. Chem. Phys. 2001, 115, 2721.
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Figure 1. Schematic model of DPN. (a) Side view and (b) 3D view. The color scheme in ODT molecules is as follows: sulfur in gray (almost invisible), CH2 in green, and CH3 in yellow.
Published on Web 10/15/2009
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Figure 2. Snapshots of MD simulation of DPN at room temperature for separation heights of (a) 2.5, (b) 3.5, (c) 4.5, and (d) 8.5 nm. The 2.5 nm separation height represents the time when the tip has indented the substrate to the specified depth.
analyzed using MD, including surface friction,13,14 nanoscratching,15 lubrication,16 contact,17 nanoindentation,18,19 and the nanotribology of self-assembled monolayer (SAM, which is ordered molecular assemblies formed by the chemical adsorption of an active surfactant on a solid surface and had the low friction, adhesion, and hydrophobic properties) behavior.20,21 Using an MD simulations of DPN, Ahn22 and Heo et al.23 studied the effect of molecule-substrate binding energy and found that increasing the moderate molecule-substrate binding energy enhances the molecular deposition rate and makes the monolayer well-ordered. In the present study, SAM molecules of 1-octadecanethiol (CH3(CH2)15SH, ODT), a prototypical molecule in DPN, are (13) Muser, M. H. Comput. Phys. Commun. 2002, 146 (1), 54. (14) Komanduri, R.; Chandrasekaran, N.; Raff, L. M. Phys. Rev. B 1997, 61(20), 14007. (15) Fang, T. H.; Weng, C. I. Nanotechnology 2000, 11, 148. (16) Capozza, R.; Fasolino, A.; Ferrario, M.; Vanossi, A. Phys. Rev. B 2008, 77, 235432. (17) Buldum, A.; Ciraci, S.; Batra, I. P. Phys. Rev. B 1998, 57, 2468. (18) Fang, T. H.; Weng, C. I.; Chang, J. G. Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. 2003, 357, 7. (19) Kelchner, C. L.; Plimpton, S. J.; Hamilton, J. C. Phys. Rev. B 1998, 58, 11085. (20) Wu, C. D.; Lin, J. F.; Fang, T. H. Comput. Mater. Sci. 2007, 39, 808. (21) Wu, C. D.; Lin, J. F.; Fang, T. H.; Lin, H. Y.; Chang, S. H. Appl. Phys. A: Mater. Sci. Process. 2008, 91, 459. (22) Ahn, Y.; Hong, S.; Jang, J. J. Phys. Chem. B 2006, 110, 4270. (23) Heo, D. M.; Yang. M.; Hwang, S.; Jang, J. J. Phys. Chem. C 2008, 112, 8791.
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considered as the deposited material. The effect of the deposition temperature is investigated using an MD simulation of the DPN process. The objectives of this study are to determine the physical behavior of molecular transport, the mechanism of pattern formation, and the mechanics characteristics of tip-SAM-substrate interactions.
2. Methodology Parts a and b of Figure 1 show the side view and 3D view of the DPN model, respectively. The model consists of a silicon (Si) tip, a gold (Au) substrate, and SAM molecules adsorbed on the tip. The diameters of molecules in the figures are not to scale. The tip had a radius of 5 nm and was assumed to be a rigid body to simplify DPN analysis. In the simulation, the tip had a constant unit displacement of 0.00001 nm per time step. The time step unit was 10-15 s. The Au substrate consists of a perfect faced-centered cubic (FCC) single crystal with a length, width, and height of 18, 15, and 2.3 nm, respectively. 2704 SAM molecules were adsorbed on the tip after an MD equilibrium run of 300 ps. A threedimensional system was simulated in the (110), (100), and (101) directions (X-, Y-, and Z-axes, respectively). A periodic boundary condition was applied to the X- and Y-axes of the Au substrate to simulate a large system by modeling a small part that is far from its edge. Two fixed layers of Au molecules were imposed beneath the substrate bottom to constrain the whole system in the vertical direction. Langmuir 2010, 26(5), 3237–3241
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Figure 4. Variation of the absorbed energy versus separation distance between the tip and substrate at temperatures of 300, 400, and 500 K.
harmonic potential. The torsional terms were assumed to have a Rychaert-Bellmans dihedral potential form, which is a power series expansion of the dihedral angle.24 The intermolecular and intramolecular nonbonding interactions for molecules separated by more than three bonds along the same chain were represented by the Lennard-Jones potential. The Lennard-Jones potential function was also employed to describe the physisorption interaction between the tip and SAM molecules. To describe molecule transfer, Luedtke et al.26 modeled the ODT-Au interaction as a Morse potential (S atoms and Au substrate have chemisorption interaction) and fitted the parameters to experimental binding energies. The motion behavior of each molecule was calculated using the Gear five-order predication method27 based on the integration of Newton’s second law. Before the DPN simulation was started, a separation distance of 1.5 nm was set between the tip (including the SAM molecules) and the substrate. For each case, the total time length of the simulation was 1 ns.
3. Results and Discussion Figure 3. Meniscus characteristic evaluation for the temperature effect. The variation of (a) the neck width in the meniscus and (b) the height versus separation distance between the tip and substrate.
The Morse potential was applied to describe the interaction for Au atoms (substrate). The potential form is U(rij) = D{exp{-2R(rij-r0)} - 2 exp{-R(rij - r0)}}. The parameters D, R, and r0 of the simulation were 0.4826 eV, 1.6166 A˚-1, and 3.004 A˚, respectively. The alkylthiol chain description proposed by Hautman and Klein24 was used in this study. The CH2 and CH3 groups were treated as a single-spherical molecule to simplify the model to 17 united molecules per chain. Because the bonds undergo distortion and the chain length decreases under high compression, the potential functions of the Hautman and Klein model were modified to allow for bond stretching and the 12-3 chain-surface.25 The bond-bending terms were modeled using a (24) Hauman, J.; Klein, M. L. J. Chem. Phys. 1989, 91, 15. (25) Tupper, K. J.; Brenner, D. W. Langmuir 1994, 10, 2335.
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3.1. Molecular Transfer Analysis in DPN Using Molecular Trajectories. Parts a-d of Figure 2 show snapshots of the molecular transfer process of the DPN simulation at room temperature. The withdraw distances of the tip for Figure 2a-d are 2.5, 3.0, 4.5, and 8.5 nm, respectively. A withdraw distance of 2.5 nm represents the moment when the tip has stopped indenting the substrate and is about to pull up, as shown in Figure 2a. At this time, very few SAM molecules at the end of the tip are directly in contact with the substrate surface. The SAM molecules are directly indented by the tip, so SAM conformations lie almost on the substrate surface. Jang et al.29 found if the tip remains to get closer to the substrate; the meniscus broadens and is stabilized. After the tip is pulled up, as shown in Figure 2b, many SAM molecules start to stand on the substrate due to pull and drag forces from the substrate and the tip. Au substrate-SAM molecules experience chemisorption while SAM-Si tip molecules (26) Luedtke, W. D.; Landman, U. J. Phys. Chem. B 1998, 102, 6566. (27) Haile, J. M. Molecular Dynamics Simulation: Elementary Methods; Wiley: New York, 1992. (28) Rozhok, S.; Piner, R.; Mirkin, C. A. J. Phys. Chem. B 2003, 107, 751. (29) Jang, J. Schatz, G. C. Ratner, M. A. J. Chem. Phys., 2002, 116, .3875.
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Figure 6. Variation of absorbed area and pattern height versus deposition temperature.
Figure 5. Deposition patterns at temperatures of (a) 300, (b) 400, and (c) 500 K. The color scheme in ODT molecules is sulfur in green, CH2 in yellow, and CH3 in red.
experience physisorption. During the pull up process shown in Figure 2b, a large number of SAM molecules are continuously transferred down the Au substrate because the chemisorption force at the Au substrate-SAM interface is much stronger than the physisorption force at SAM-Si tip interface. With increasing withdraw distance, an obvious meniscus of the SAM forms at the interface of the substrate and the tip, as shown in Figure 2, parts c and d. Interestingly, once the meniscus forms, the neck width of its cross section gradually decreases with increasing withdrawal distance of the tip until the meniscus breaks. At the withdraw distance of 8.5 nm (shown in Figure 2d), the meniscus would be broken. The interaction force field near the position of the break belongs to the weak nonbonding action (SAM-SAM). 3.2. Molecular Transfer Analysis in DPN Using the SAM Meniscus Characteristic. A clear understanding of the meniscus variation is important in the DPN process because the meniscus is considered as a channel for the molecules to move from the tip to the substrate. Humidity variation changes the meniscus size in experiments1,28 and affects monolayer growth. 3240 DOI: 10.1021/la9029112
Moreover, the meniscus width decreases dramatically when tip curvature was increased.29 Parts a and b of Figure 3 show the variations of the neck width in meniscus and the height versus the separation distance between the tip and substrate at temperatures of 300, 400, and 500 K. The meniscus characteristic was evaluated every 5 ps. The value of the neck width in the meniscus was decided by comparing it to the neck width of the cross area of the meniscus in both the X-Z and Y-Z planes. Parts a and b of Figure 3 show a similar trend of the temperature effect. The neck width in meniscus decreased when the separation distance between the tip and substrate increased. This can be explained by molecular thermal vibration. At the initial withdraw stage for the neck width variation in the meniscus in Figure 3a, the oscillation is proportional to the temperature increase. A large oscillation indicates that the meniscus has just formed and that the structure of the meniscus was unstable. After the initial withdraw stage, the neck width in the meniscus had only a slight variation. When the deposition temperature was increased, the withdraw distance could be longer before the meniscus broke. 3.3. Molecular Transfer Analysis in DPN Using Absorbed Energy with Au Substrate-SAM. Figure 4 shows the variation of absorbed energy versus separation distance of the tip and substrate in the DPN process at temperatures of 300, 400, and 500 K. The absorbed energy represents the sum of the surface energy from the Au surface interacting with SAM molecules. The magnitude of the absorbed energy is highest at the end of the initial indentation, with the value (about -150 eV) being almost the same for the three temperatures. This indicates that the molecular transfer rate is stable and that the number of adsorbed molecules was very similar at the three temperatures during this first transfer stage. A large energy decay can be clearly observed at the withdraw distance of 1-2 nm (second transfer stage). The speed of decay and the critical diffuse time (which is related to the magnitude and length increased with increasing deposition temperature due to higher kinetic energy. The smaller absorbed energy indicates that more molecules were adsorbed on the substrate. The second transfer stage is a critical period for determining how many SAM molecules can be continuously transferred onto the substrate by thermal diffusion motion after the first transfer stage. The multiple values Langmuir 2010, 26(5), 3237–3241
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indicate that the deposition temperature is a key parameter in the DPN process. The transfer rate increased 0.8-, 2-, and 4-fold for temperatures of 300, 400, and 500 K, respectively. After the second transfer stage, the oscillation in all curves became small. This indicates that the amount transferred from the tip was very small. 3.4. Molecular Transfer Analysis in DPN Using Deposition Patterns. Parts a-c of Figure 5 show a series of deposition patterns at temperatures of 300, 400, and 500 K, respectively. The pattern characteristics are strongly related to the deposition temperature. The pattern shape was more concentrated and the area was smaller when the pattern was deposited at room temperature. At higher temperatures, the pattern shape expanded and the area increased because of good molecular diffusion. In the three patterns, the numbers of SAM molecules were 270, 344, and 455, respectively. The result shows a better transferred ability at higher temperature and conform to experiment results.1,28 In general, except the factor of temperature, the transferred ability also obvious increase with increasing dwell time for most deposited material.1,11,28,30 Figure 6 shows the relationship of absorbed area and pattern height versus deposition temperature. The magnitude of the absorbed area is proportional to the deposition temperature. However, there is no clear relationship between pattern height and deposition temperature. The pattern height depends on dwell time time.1 (30) Weeks, B. L. Noy, A., Miller, A. E. De Yoreo, J. J. Phys. Rev. B. 2002, 88, 255505.
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4. Conclusion The MD simulation revealed the pattern formation mechanism and mechanics in the DPN process. The following conclusions were obtained: (1) At the first transfer stage (at the initial indentation depth), the conformation of SAM molecules lies almost on the substrate surface. The molecules start to stand on the substrate due to the pull and drag forces at the second transfer stage (after the tip was pulled up). (2) The average neck width in the meniscus decreased and the height increased with increasing separation distance between the tip and substrate. (3) According to the simulation snapshots (Figures 2 and 5) and absorbed energy evaluation (Figure 4), the number of deposited molecules in the first transfer stage is low. The second stage has larger transfer amount and the rate is strongly depended on the transfer temperature. (4) When molecules were deposited at a low temperature (e.g., room temperature), the pattern shape was more concentrated and the area was smaller. At higher temperatures, the pattern shape expanded and the area increased because of good molecular diffusion. Acknowledgment. This work was supported in part by the National Science Council of Taiwan under grants NSC 97-2221E-150-069-MY3 and NSC 098-2811-E-150-001.
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